Kofron et al U.S. Pat. No. 4,439,520 ushered in the current era of high performance silver halide photography. Kofron et al disclosed and demonstrated striking photographic advantages for chemically and spectrally sensitized tabular grain emulsions in which tabular grains having a diameter of at least 0.6 .mu.m and a thickness of less than 0.3 .mu.m exhibit an average aspect ratio of greater than 8 and account for greater than 50 percent of total grain projected area. In the numerous emulsions demonstrated one or more of these numerical parameters often far exceeded the stated requirements. Kofron et al recognized that the chemically and spectrally sensitized emulsions disclosed in one or more of their various forms would be useful in color photography and in black-and-white photography (including indirect radiography). Spectral sensitizations in all portions of the visible spectrum and at longer wavelengths were addressed as well as orthochromatic and panchromatic spectral sensitizations for black-and-white imaging applications. Kofron et al employed combinations of one or more spectral sensitizing dyes along with middle chalcogen (e.g., sulfur) and/or noble metal (e.g., gold) chemical sensitizations, although still other, conventional sensitizations, such as reduction sensitization were also disclosed.
In 1982 the first indirect radiographic and color photographic films incorporating the teachings of Kofron et al were introduced commercially. Now, 12 years later, there are clearly understood tabular grain emulsion preferences that are different, depending on the type of product being considered. Indirect radiography has found exceptionally thin tabular grain emulsions to be unattractive, since they produce silver images that have an objectionably warm (i.e., brownish black) image tone. In camera speed color photographic films exceptionally thin tabular grain emulsions usually have been found attractive, particularly when spectrally sensitized to wavelength regions in which native grain sensitivity is low--e.g., at wavelengths longer than about 430 nm. Comparable performance of exceptionally thin tabular grain emulsions containing one or more spectral sensitizing dyes having an absorption peak of less than 430 nm is theoretically possible. However, the art has usually relied on the native blue sensitivity of camera speed emulsions to boost their sensitivity, and this has retarded the transition to exceptionally thin tabular grain emulsions for producing blue exposure records. Grain volume reductions that result from reducing the thickness of tabular grains work against the use of the native blue sensitivity to provide increases in blue speed significantly greater than realized by employing blue absorbing spectral sensitizing dyes. Hence, thicker tabular grains or nontabular grains are a common choice for the blue recording emulsion layers of camera speed film.
Recently, Antoniades et al U.S. Pat. No. 5,250,403 disclosed tabular grain emulsions that represent what were, prior to the present invention, in many ways the best available emulsions for recording exposures in color photographic elements, particularly in the minus blue (red and/or green) portion of the spectrum. Antoniades et al disclosed tabular grain emulsions in which tabular grains having {111} major faces account for greater than 97 percent of total grain projected area. The tabular grains have an equivalent circular diameter (ECD) of at least 0.7 .mu.m and a mean thickness of less than 0.07 .mu.m. Tabular grain emulsions with mean thicknesses of less than 0.07 .mu.m are herein referred to as "ultrathin" tabular grain emulsions. They are suited for use in color photographic elements, particularly in minus blue recording emulsion layers, because of their efficient utilization of silver, attractive speed-granularity relationships, and high levels of image sharpness, both in the emulsion layer and in underlying emulsion layers.
A characteristic of ultrathin tabular grain emulsions that sets them apart from other tabular grain emulsions is that they do not exhibit reflection maxima within the visible spectrum, as is recognized to be characteristic of tabular grains having thicknesses in the 0.18 to 0.08 .mu.m range, as taught by Buhr et al, Research Disclosure, Vol. 253, Item 25330, May 1985. Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley House, 12 North St., Emsworth, Hampshire P010 7DQ, England. In multilayer photographic elements overlying emulsion layers with mean tabular grain thicknesses in the 0.18 to 0.08 .mu.m range require care in selection, since their reflection properties differ widely within the visible spectrum. The choice of ultrathin tabular grain emulsions in building multilayer photographic elements eliminates spectral reflectance dictated choices of different mean grain thicknesses in the various emulsion layers overlying other emulsion layers. Hence, the use of ultrathin tabular grain emulsions not only allows improvements in photographic performance, it also offers the advantage of simplifying the construction of multilayer photographic elements.
An early, cross-referenced variation on the teachings of Kofron et al was provided by Maskasky U.S. Pat. No. 4,435,501, hereinafter referred to as Maskasky I. Maskasky I recognized that a site director, such as iodide ion, an aminoazaindene, or a selected spectral sensitizing dye, adsorbed to the surfaces of host tabular grains was capable of directing silver salt epitaxy to selected sites, typically the edges and/or corners, of the host grains. Depending upon the composition and site of the silver salt epitaxy, significant increases in speed were observed. The most highly controlled site depositions (e.g., corner specific epitaxy siting) and the highest reported photographic speeds reported by Maskasky I were obtained by epitaxially depositing silver chloride onto silver iodobromide tabular grains. Maskasky I did not have available an ultrathin tabular grain emulsion to sensitize by epitaxial deposition, but it is clear that had such emulsion been available the intentional introduction of iodide during epitaxial deposition would not have been undertaken. Maskasky I taught a preference for epitaxially depositing a silver salt having a higher solubility than the host tabular grains, stating that this reduces any tendency toward dissolution of the tabular grains while silver salt is being deposited. It would appear intuitively obvious that ultrathin tabular grains would be more susceptible to dissolution than the much thicker tabular grains that Maskasky I actually employed in its reported investigations. Maskasky I recognized that even when chloride is the sole halide run into a tabular grain emulsion during epitaxial deposition, a minor portion of the halide contained in the host tabular grains can migrate to the silver chloride epitaxy. Maskasky I offers as an example the inclusion of minor amounts of bromide ion when silver and chloride ions are being run into a tabular grain emulsion during epitaxial deposition. From the iodide levels contained in the tabular grain emulsions of Maskasky I and the investigations of this invention, reported in the Examples below, it is apparent that the epitaxial depositions of Maskasky I contained only a fraction of a mole percent iodide transferred from the host tabular grains.
Maskasky U.S. Pat. No. 4,471,050, hereinafter referred to as Maskasky II, discloses that nonisomorphic silver salts can be selectively deposited on the edges of silver halide host grains without relying on a supplemental site director. The nonisomorphic silver salts include silver thiocyanate, .beta. phase silver iodide (which exhibits a hexagonal wurtzite type crystal structure), .gamma. phase silver iodide (which exhibits a zinc blende type crystal structure), silver phosphates (including meta- and pyro-phosphates) and silver carbonate. None of these nonisomorphic silver salts exhibit a face centered cubic crystal structure of the type found in photographic silver halides--i.e., an isomorphic face centered cubic crystal structure of the rock salt type. In fact, speed enhancements produced by nonisomorphic silver salt epitaxy have been much smaller than those obtained by comparable isomorphic silver salt epitaxial sensitizations.